Voltage Collapse

[vtpower]

We recently had a very large fault on one of our 34.5kV Subtransmission lines. It was a phase to phase fault (no ground involved), and the fault itself was about 11,000 Amps. As you can imagine there was contribution from a large number of sources, as it is a looped system. At one of the remote substations, approximately 8 miles away, there was a contribution up a different transmission line of about 1,000 Amps. This breaker should not have tripped, but after downloading the Schweitzer data, we noticed that the bus voltage on all three phases collapsed to nearly 0! This made it look like a Zone 1 fault and caused the breaker to trip. The question is, what would have caused all three phases to collapse with a phase to phase fault??? The bus PT's are connected Wye Grounded. All sources to this substation remained in and as I mentioned the contribution from this substation to the fault was rather low (1,000 amps). Let me know if my description is unclear. What makes matters worse is that we still can't find what the heck caused the fault. Thanks for the help.

 

[dpc]

Are there three PTs?

 

[vtpower]

Yes, there are three PT's connected phase to ground.

 

[gobblerhuntr]

I just went to a SEL fault analysis seminar and got a lot out of it as far as d/ling data and reading it. I have a name and contact number from the guy that put it on and I bet he would be glad to look at it if you would send him your data. If you are interested let me know and I will get his info Monday and give it to you. What part of the country are you in?

 

[vtpower]

Thanks, I am in the Northeast. I usually deal with George A. from SEL, but would greatly appreciate any contacts you may have to dig into this.

Thanks

 

[swn1]

Just because there are three PTs doesn't mean they are all being read. Make sure the relay knows there are three!

Yes, it happens.

 

[dpc]

You've probably resolved this by now - but I'd be interested in the answer. I assume you verified that the relay is reading voltage on all three phases during normal operation?

dpc

 

[vtpower]

dpc, As it turned out, once upon a time, someone had a the idea of inserting an automatic throwover scenario between two seperate sets of bus pots. In my exploration of the AC, DC schematics, I failed to recognize this. It turns out the relay that would do the transfer was dropping out and essentially opening the pot circuits. The only thing I am still unsure of, is the SEL are designed to block diretional tripping upon loss of potential so I am not sure why the trip was still issued unless the timing was to close and it was too late to block tripping.

 

[JBinCA]

Perhaps the blocking function only prevents the initiation of timing-out on Directional current, but does not prevent it from continuing to time out if the block signal is lost after pick-up but before time-out.

SEL has first rate tech support and the would be abble to answer that question.

Regards,

JB